Lemma 3  [1]} If \(A=(a_{nk})_{n,k=0}^{\infty }\) , then
\(A\in (\ell _{\infty },c)\)  if and only if condition (REF ) holds and
\( \lim _{n\rightarrow \infty } \sum _{k=0}^{\infty }|a_{nk}|=\sum _{k=0}^{\infty } \left|\lim _{n\rightarrow \infty } a_{nk}\right|\) 

